The present invention relates to a bus regulating system employing a so-called daisy chain. More particularly, the present invention is directed to a bus regulating system for changing bus usage priority with software.
Heretofore a daisy chain bus regulating system has been used. One example of such a bus regulating system is shown in FIG. 4 of the accompanying drawings. Designated in FIG. 4 is a bus regulating circuit 1 for determining a module that is permitted to use a bus, a bus 2, and a request signal line 3 for transferring a request signal *RQ by which each module requests use of the bus 2. A bus usage permit signal line 4 transfers a bus usage permit signal *BG issued by the bus regulating circuit 1 for permitting each module to use the bus 2. The bus 2 is shared by modules 10, 20 and 30. (In the present description, a signal which is a logic "0" when it is effective is associated with the sign "*" in front of the alphabetical letters that denote the signal.)
In FIG. 4, when one of the modules issues a bus usage request signal *RQ, the bus regulating circuit 1 sends a bus usage permit signal *BG to the line 4 if the bus is not occupied. Each module receives the bus usage permit signal *BG and exclusively uses the bus if it has issued the request signal *RQ. If a module has not issued the request signal *RQ, then it transfers the bus usage permit signal *BG to the next module.
In conventional systems, when the modules 10, 20 and 30 shown in FIG. 4 simultaneously issue request signals *RG, since the bus regulating system is in the form of a daisy chain, those modules closer to the left have higher bus usage priority and those closer to the right have lower bus usage priority. Therefore, bus usage priority is determined by the hardware arrangement, and a module which is processing a task with higher priority may not necessarily be allowed to use the bus with priority.